Synchronization of a decoding and presentation process for received bitstreams is a particularly important aspect of real-time digital data communication and delivery systems. Because received data is expected to be processed at a particular rate (to match the rate at which it is generated and transmitted), loss of synchronization leads to either buffer overflow or underflow at the decoder, and as a consequence, loss of presentation and/or displaced synchronization.
In several communications systems the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference.
MPEG-compliant systems are being used to generate and store complete presentations containing video, audio and ancillary data. Since the amount of data generated for each picture is variable (based on the picture coding approach and complexity), timing cannot be derived directly from the start of picture data. Therefore, timing signals that are crucial to the proper display of the presentation are embedded in the MPEG information stream at the time of generation or storage. These timing signals are used to accurately display the information at the receive side of an MPEG-compliant system.
Current analog television studio practice is to synchronize incoming signals to the studio reference timing. This is accomplished through the use of frame synchronizers, which add or delete frames based upon the differences in the timing references at the signal source and destination. Analog television system frame synchronizers utilize synchronizing pulses included in the analog television signal. These sync pulses delineate horizontal lines, picture fields and picture frames.
Since an MPEG-compliant information stream does not include analog television type sync pulses, frame synchronization of such a stream presents a uniquely different problem. For example, an MPEG-compliant transmitter station encodes a program and transmits the encoded program as a television signal which is referenced to a local (i.e., transmitter) oscillator, such as a 27 MHz station clock. A receiver station having its own local oscillator receives and decodes the television signal. The receiver must synchronize the received signal to the local timing reference. One method is to synchronize the send and receive clocks. This is costly and disadvantageously requires that one of the stations (i.e., transmit or receive) relinquish control of the station clock to the other station or to a reference outside of the station.
Therefore, a need exists in the art for a cost-efficient method and apparatus for synchronizing video and other compressed information frames included within a compressed information stream such as an MPEG-compliant information stream. Is also seen to be desirable to provide a frame synchronizing method and apparatus in which local control of timing references is retained while providing the necessary frame synchronization between the frame rate of the transmitter station and the frame rate of the receive station.